Seedorf capitalised on confusion in the United defence to fire in a second after half an hour.
And substitute Alberto Gilardino broke away to wrap up an easy win as United limped out in disappointing fashion.
Brazilian hero Kaka the main star of the match
The result ended United's dreams of repeating their 1999 Treble, but they can have no complaints after being totally outclassed by their Italian opponents.
Sir Alex Ferguson named Nemanja Vidic in his starting line-up after the defender's recovery from a broken collarbone but the Serb, like his colleagues, was overwhelmed by Milan's superior passing and movement in wet conditions.
The thunderous skies only added to the intimidating atmosphere at the San Siro, and when Milan began at a blistering pace, United were simply awestruck.
The hosts threatened in the very first minute when Kaka delivered a dangerous ball across goal, after three minutes when Seedorf forced Edwin van der Sar to tip over and again on seven minutes when Kaka shot straight at the United keeper.
The goal, then, was no more than Milan deserved, Seedorf nodding the ball down for Kaka to hammer home his 10th Champions League goal of the season.
Wayne Rooney endured a frustrating night at the San Siro
United's much-vaunted attack was proving toothless.
Cristiano Ronaldo, who started on the left, was hardly in the game, Ryan Giggs was similarly anonymous, while Wayne Rooney's touch deserted him.
The England striker was guilty of giving the ball away on numerous occasions but at least his errors were nowhere near as catastrophic as Gabriel Heinze's moment of madness.
As United tried to play their way out of defence, the left-back inexplicably passed back to Vidic, who then compounded his team-mate's error by slipping as he scrambled to clear.
Milan regained possession and the ball eventually fell for Seedorf, who rode Vidic's challenge before finding the same corner as Kaka.
At half-time, optimistic United fans might have pointed to their side's recovery from two goals down in the 1999 semi-final against Juventus in Turin, but Milan were simply too good for such lacklustre opponents.
United had far more possession in the second half but rarely threatened a calm and composed Milan defence nor did they manage to test Milan's out-of-form keeper Dida.
Rooney finally found an accurate pass to set up Darren Fletcher, but he skewed wide and that was as good a chance as United fashioned.
And Milan booked a repeat of the 2005 final against Liverpool when Gilardino galloped through a gaping hole in the United defence and stroked the ball past a stranded Van der Sar.
AC Milan: Dida, Oddo, Nesta, Kaladze, Jankulovski, Gattuso (Cafu 84), Pirlo, Seedorf, Ambrosini, Kaka (Favalli 86), Inzaghi (Gilardino 66).
Subs Not Used: Kalac, Bonera, Serginho, Brocchi.
Booked: Ambrosini, Gattuso.
Goals: Kaka 11, Seedorf 30, Gilardino 78.
Man Utd: Van der Sar, O'Shea (Saha 77), Brown, Vidic, Heinze, Ronaldo, Fletcher, Scholes, Carrick, Giggs, Rooney.
Subs Not Used: Kuszczak, Ferdinand, Smith, Solskjaer, Richardson, Eagles.
Booked: Ronaldo.
Att: 78,500
Ref: Frank De Bleeckere (Belgium).
Sir Alex Ferguson named Nemanja Vidic in his starting line-up after the defender's recovery from a broken collarbone but the Serb, like his colleagues, was overwhelmed by Milan's superior passing and movement in wet conditions.
The thunderous skies only added to the intimidating atmosphere at the San Siro, and when Milan began at a blistering pace, United were simply awestruck.
The hosts threatened in the very first minute when Kaka delivered a dangerous ball across goal, after three minutes when Seedorf forced Edwin van der Sar to tip over and again on seven minutes when Kaka shot straight at the United keeper.
The goal, then, was no more than Milan deserved, Seedorf nodding the ball down for Kaka to hammer home his 10th Champions League goal of the season.
Wayne Rooney endured a frustrating night at the San SiroUnited's much-vaunted attack was proving toothless.
Cristiano Ronaldo, who started on the left, was hardly in the game, Ryan Giggs was similarly anonymous, while Wayne Rooney's touch deserted him.
The England striker was guilty of giving the ball away on numerous occasions but at least his errors were nowhere near as catastrophic as Gabriel Heinze's moment of madness.
As United tried to play their way out of defence, the left-back inexplicably passed back to Vidic, who then compounded his team-mate's error by slipping as he scrambled to clear.
Milan regained possession and the ball eventually fell for Seedorf, who rode Vidic's challenge before finding the same corner as Kaka.
At half-time, optimistic United fans might have pointed to their side's recovery from two goals down in the 1999 semi-final against Juventus in Turin, but Milan were simply too good for such lacklustre opponents.
United had far more possession in the second half but rarely threatened a calm and composed Milan defence nor did they manage to test Milan's out-of-form keeper Dida.
Rooney finally found an accurate pass to set up Darren Fletcher, but he skewed wide and that was as good a chance as United fashioned.
And Milan booked a repeat of the 2005 final against Liverpool when Gilardino galloped through a gaping hole in the United defence and stroked the ball past a stranded Van der Sar.
AC Milan: Dida, Oddo, Nesta, Kaladze, Jankulovski, Gattuso (Cafu 84), Pirlo, Seedorf, Ambrosini, Kaka (Favalli 86), Inzaghi (Gilardino 66).
Subs Not Used: Kalac, Bonera, Serginho, Brocchi.
Booked: Ambrosini, Gattuso.
Goals: Kaka 11, Seedorf 30, Gilardino 78.
Man Utd: Van der Sar, O'Shea (Saha 77), Brown, Vidic, Heinze, Ronaldo, Fletcher, Scholes, Carrick, Giggs, Rooney.
Subs Not Used: Kuszczak, Ferdinand, Smith, Solskjaer, Richardson, Eagles.
Booked: Ronaldo.
Att: 78,500
Ref: Frank De Bleeckere (Belgium).
TOTW/F/M Award(s): 2
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are dukkher kotha abaro mone koraila ken??
the justice of the night, i'm dark knight 
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